RFID systems are well known in the art. Such systems include relatively large packages containing battery powered transmission/receiving circuitry, such as the identification system disclosed in U.S. Pat. No. 4,274,083, to passive systems in which the transponder receives its power from the base station or interrogator, such as the identification system disclosed in U.S. Pat. No. 4,654,658.
A typical RFID system is made up of reusable transponders or tags fixed to or embedded in product carriers, an antenna system that interrogates the tags via a RF link and a controller. The host (or computer) system interfaces with the controller and directs the interrogation of the tags.
The RFID antenna system typically employs a high frequency signal to interrogate the tags and, hence, product carriers which are moved on an interrogation path, such as a conveyor. The antenna system is generally disposed near the interrogation path to provide effective communication to and from the tags.
The location of the antenna is critical to the interrogation and receipt of the identification code and other data transmitted to and from the tags. For example, if a relatively small single antenna is employed or the antenna is positioned too far from the interrogation path, inadequate coverage of the interrogation path will occur.
Several attempts have been made to optimize the RF link and, hence, communication between the antenna and the tags. In one (typically employed) approach, a frame antenna of rectangular or square shape is fitted around the conveyor.
The noted system has several drawbacks. In particular, the antenna system exhibits one or more dead zones for certain tag positions. For example, there is a dead zone across the very center of the conventional frame antenna. If a tag is located within a window which is parallel to the conveyor belt and comprises some distance on either side of the center frame axis, and maintains this position throughout the read area of the antenna, the tag would not be read.
Moreover, the accuracy and completeness of the reading also decreases if several tags are following one another in close succession during movement along the conveyor. When tags are in close succession, and relatively far from the read antenna, the tags appear to be the same distance from the read antenna and thus send back simultaneous transmissions. The result of a simultaneous transmission is an unintelligible identification code. This is particularly the case in a noisy environment, for which shielding would be necessary.
It is, therefore, an object of the present invention to provide a RFID antenna system which provides optimum coverage over the interrogation path.
It is another object of the present invention to provide a RFID antenna which is readily incorporated into a conveyor system to provide accurate interrogation of tags fixed to or embedded in product carriers.